Regularly taking a moment to stop and show your partner you're thankful for her small acts of kindness — be they taking out the trash or fixing your computer — can make both of you feel more satisfied and strengthen your relationship.

Psychologists didn't start systematically studying gratitude— let alone its impact on romantic relationships— until the early 2000s. Before then, most of the research in the field focused on negative emotions and the problems that either produced or stemmed from these feelings.

But a decade of social science research suggests that partners who show they care about the little things activate a two-way feedback system that helps both members of a relationship feel closer and more fulfilled.

Two psychologists, University of California, Davis' Robert Emmons and University of Miami's Michael McCullough spearheaded most of the early research on gratitude's effects.

In one of of their studies, the researchers had volunteers keep weekly journals in which they wrote about particular topics. One group wrote about major events that had happened that week. Another group wrote about hassles they'd experienced. The last group wrote about things they were grateful for. Ten weeks later, those in the gratitude group reported feeling more optimistic and more satisfied with their lives than those in any of the other groups. They also reported fewer physical symptoms of discomfort, from runny noses to headaches, and exercised more.

Years later, University of North Carolina at Chapel Hill psychologist Sara Algoe took those same feelings of gratefulness and studied how they might affect not just one person, but couples in romantic relationships. For her study, Algoe also had couples keep a diary (just like Emmons and McCullough had). Instead of recording anything they felt grateful for, however, Algoe had her participants list things their partner had done that made them feel grateful, along with how each act of kindness made them feel. Participants also kept track of kind acts they directed toward their partner, and how those made them feel.

Over the course of 1,768 days of reports, participants reported that their partner did something thoughtful for them nearly 700 times, while they reported doing something thoughtful for their partner slightly less often (601 times). But there was a sad twist: Nearly half of the attempted acts of kindness went undetected by the other person. What mattered, it turned out, wasn't how often someone in the relationship did a thoughtful thing — it was how grateful the partner reported feeling about it.

Volunteers were more connected to their partners and more satisfied with their relationship on the days when they reported feeling more grateful for their partners' acts of kindness. And those feelings of gratitude — more important than any acts of kindness alone — lasted into the following day.

Couples who took a moment to show they cared about their partner's efforts got a temporary mood boost. But showing gratitude, it turns out, can also have a lasting impact on relationships.

In a series of studies, University of California, Berkeley psychologist Amie Gordon found that the more grateful couples were, the more likely they were to still be in the relationship nine months down the road.

Gordon's research had one crucial caveat, though: Expressing gratitude isn't confined solely to saying "thank you" for a kind deed. Being grateful, she writes in a blog post for Psychology Today,is about feeling lucky to have a caring partner in the first place. "My definition of gratitude includes appreciating not just what your partner does, but who they are as a person. You're not just thankful that your partner took out the trash — you're thankful that you have a partner who is thoughtful enough to know you hate taking out the trash."

Happy couples can make it seem like it all comes naturally, but in reality any strong, quality relationship requires a hefty amount of work. If you want to make your relationship stronger — and you're willing to put in the effort — gratitude itself can help you and your partner feel happier and more connected.

All of this is based on the idea that gratitude itself can generate more positive thinking. Recent research, including dozens of studies done in individuals and couples, appears to back up this notion. Of course, while gratitude can be used to help strengthen a healthy relationship, it shouldn't be used as a means of justifying staying in an unhealthy one.

Gratitude is contagious, Gordon's research found. It produces a cascade of feelings and behaviors, many of which also happen to be critical to strengthening a relationship. When you actively think about your partner's caring qualities, you begin to think about how much he or she means to you.

Partners who regularly think of each other as valuable and important, it turns out, behave in ways that reflect those feelings.

Imagine stopping for a moment to think about the last time your girlfriend took care of you when you were sick. The next time she says something to you, you might listen more carefully, either because you want to return the favor or because you've actively made yourself aware of how important she is to you.

Either way, she'll likely notice, and she'll feel more appreciated as a result.

When someone feels appreciated by their partner, they in turn appreciate the partner more too, creating something of a happy cycle. In long-term relationships, it's those simple behaviors that can make all of the work worthwhile.

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